1. Field of the Invention
The present invention relates to an apparatus and a method for driving a self-luminescent display panel that performs gradation expression by time-dividing one frame period into a plurality of subframe periods, and controlling the lighting of each subframe period, as well as to an electronic appliance equipped with the driving apparatus.
2. Description of the Related Art
Development of displays using a display panel constituted by arranging luminescent elements in a matrix form is widely proceeding. As a luminescent element used for such a display panel, an organic EL (electroluminescence) element using an organic material in a luminescent layer, for example, is attracting people's attention.
As a display panel using such an organic EL element, there is an active matrix type display panel in which an active element made of a TFT (thin film transistor), for example, is added to each of the EL elements arranged in a matrix form. This active matrix type display panel can realize low electric power consumption, and also has properties such as less cross-talking between the pixels, so that it is suitable for a highly fine display constituting a large screen.
FIG. 1 shows one example of a circuit construction corresponding to one pixel 10 in a conventional active matrix type display panel. In FIG. 1, the gate G of a TFT 11, which is a transistor for control, is connected to a scanning line (scanning line A1), and the source S is connected to a data line (data line B1). Also, the drain D of this TFT 11 for control is connected to the gate G of a TFT 12, which is a transistor for driving, and is also connected to one terminal of a capacitor 13 for holding electric charge.
The drain D of the TFT 12 for driving is connected to the other terminal of the capacitor 13, and is connected to a common anode 16 formed within the panel. The source S of the TFT 12 for driving is connected to the anode of an organic EL element 14, and the cathode of this organic EL element 14 is connected to a common cathode 17 formed within the panel and constituting a standard potential point (ground), for example.
FIG. 2 is a model view showing a state in which the circuit constructions that are in charge of the pixels 10 shown in FIG. 1 are arranged on a display panel 20. At each of the intersection positions of the scanning lines A1 to An and the data lines B1 to Bm, each pixel 10 having a circuit construction shown in FIG. 1 is respectively formed. In the above-described construction, the drains of the TFT 12 for driving are connected to the common anode 16 shown in FIG. 2, and the cathodes of the EL elements 14 are connected to the common cathode 17 shown in FIG. 2. In performing a luminescence control in this circuit, a switch 18 is brought into a state of being connected to the ground, as shown in FIG. 2, whereby a voltage source +VD is supplied to the common anode 16.
When an on-voltage is supplied via a scanning line to the gate G of a TFT 11 for control in FIG. 1 in this state, the TFT 11 passes, from the source S to the drain D, an electric current corresponding to the voltage from the data line that is supplied to the source S. Therefore, during the period in which the gate G of the TFT 11 is at the on-voltage, the aforesaid capacitor 13 is charged, and that voltage is supplied to the gate G of the TFT 12 for driving. The TFT 12 passes an electric current based on the gate voltage and the drain voltage from the source S through the EL element 14 to the common cathode 17, so as to make the EL element 14 luminescent.
When the gate G of the TFT 11 is brought to an off-voltage, the TFT 11 will be in a so-called cut-off state, and the drain D of the TFT 11 will be in an open state. However, in the TFT 12 for driving, the electric charge stored in the capacitor 13 holds the voltage of the gate G and maintains the driving current till the next scanning, whereby the luminescence of the EL element 14 is also maintained. Here, since a gate input capacitance is present in the aforementioned TFT 12 for driving, an operation similar to the above-described one can be performed even if the aforesaid capacitor 13 is specially provided.
In the meantime, as a system that performs gradation display of image data by using a circuit construction such as described above, there is a time gradation system. This time gradation system is a system in which, for example, one frame period is time-divided into a plurality of subframe periods, and a half-tone (intermediate gradation) display is carried out by an accumulated sum of the subframe periods in which the organic EL element emitted light per one frame period.
Further, in this time gradation system, there are a system (which is referred to as simple subframe method for the sake of convenience) in which the EL element is made to emit light subframe by subframe, and the gradation expression is carried out by a simple accumulated sum of the luminescent subframe periods, as shown in FIG. 3, and a system (which is referred to as weighted subframe method for the sake of convenience) in which, by treating one or plural subframe periods as one set, the gradation bit is allotted to the set for weighting, and the gradation expression is carried out by a combination thereof, as shown in FIG. 4. Here, in FIGS. 3 and 4, an example is shown in which eight gradations from gradation 0 to gradation 7 are displayed.
Among these, the weighted subframe method provides an advantage in that a multiple gradation display can be realized with a smaller number of subframes than in the simple subframe method by performing weighting control for gradation display also to the lighting period in a subframe period. However, in this weighted subframe method, the gradation is expressed by a combination of luminescences that are discrete in the time direction on an image of one frame, so that a contour-like noise, which is called a pseudo-moving-picture outline noise (hereafter also referred to simply as pseudo outline noise) may be generated, and this is one factor for image quality deterioration. This pseudo-outline noise will be described with reference to FIG. 5. FIG. 5 is a view for describing a mechanism of pseudo-outline noise generation. In FIG. 5, description will be made by raising an example in which four sets of subframes (set 1 to set 4) that are weighted to the brightnesses of the powers of two (weight 1, 2, 4, 8) are arranged in the order of increasing brightness.
An image having a brightness elevated by one step pixel by pixel as it goes downwards in the display screen, namely an image with gradually changing brightness, is considered. Assume that this image goes upward for the distance of one pixel after one frame passes. As illustrated, frame 1 and frame 2 are shifted by one pixel in the display position on the screen. However, a human eye cannot perceive the discrepancy of this image movement.
However, since a human eye has a property of following a moving brightness, the eye follows a set of subframes that are not luminescent at a position between the brightness 7 and the brightness 8 at which the luminescence pattern changes greatly by carriage of digits, for example, so that the human eye perceives as if a black pixel having a brightness 0 is moving. Therefore, the human eye recognizes a brightness that is not inherently present, and this is perceived as a contour-like noise. Thus, in displaying the same gradation data at the same pixel in consecutive frames, the pseudo-outline noise is likely to be generated if the luminescence pattern in each frame is the same.
As a countermeasure to cope with such a problem, there is a method of changing the order of display of the sets of weighted subframes frame by frame. In the example shown in FIG. 6, in each of the two consecutive frames (referred to as the first frame and the second frame), the order of display of the weighted sets is made different. In other words, in the first frame, the display is made in the order of the sets of weight 4, weight 2, weight 1, whereas in the second frame, the display is made in the order of the sets of weight 1, weight 4, weight 2. This makes the luminescence pattern be different even with the same gradation data in consecutive frames, thereby restraining the generation of pseudo-outline noise to some extent.
Here, a gradation display having a devised luminescence pattern of one frame data for restraining the generation of pseudo-moving-picture outline noise is disclosed, for example, in Japanese Patent Application Laid-Open (JP-A) No. 2001-125529 (page 3, right column, line 45 to page 4, left column, line 9, FIG. 2) also.
In the method shown in FIG. 6, control is made so that the luminescence pattern may be different between consecutive frames in the same pixel, so that the perception of the pseudo-outline noise by human vision sense can be reduced to some extent. However, even if any devising is made, there will be no change in the principle of gradation expression by a combination of the luminescences that are discrete in the time direction in the weighted subframe method, so that it is not possible to restrain the generation of the pseudo-outline noise completely.
On the other hand, in the simple subframe method, the luminescence in plural subframe periods are not largely discrete in the luminescence of one frame period, so that the generation of pseudo-outline noise can be restrained to some extent. However, in the simple subframe method, gradation display is made by letting one or plural consecutive subframe periods be simply luminescent, so that one frame period must be divided into many subframe periods for realizing a multiple gradation display. In that case, the clock frequency must be set high, thereby raising a problem in that the load imposed upon the driving peripheral circuit becomes large.
Also, since an organic EL element is a current injection type luminescent element, the electric current that flows through the wiring resistance imposed upon the element is largely dependent on the ratio of lighting of the luminescent display panel. Namely, when a change is made to increase the ratio of lighting greatly, the amount of voltage fall of the wiring resistance increases, thereby generating a phenomenon such that the driving voltage of the element decreases and the luminescence brightness decreases. This phenomenon is more liable to occur in the weighted subframe method in which the ratio of lighting tends to change rapidly. In this case, there will be a problem in that the gradation display is destroyed, making it impossible to perform a normal gradation expression (generation of gradation abnormality).